I don´t know if it was double bounce. Looking at the video its difficult to see and the way it was I think if it touched the ground probably João really believed he reached the ball before second bounce. Melzer wasn´t expecting the ball to be returned after the drop shot thats why he wasn´t ready but for a shot that was so difficult to judge Melzer should be ready to cover the net instead of stopping.
Honestly anyone who actually plays tennis knows that sometimes it's just that close, and you were so intensely focused on getting there you don't even know for sure whether the ball touched the ground or not. Melzer is way too experienced to take things like this so personally.